1. Field of the Invention
The present invention relates to a quartz glass substrate for a substrate having fine convexoconcave patterns on its surface. More particularly, it relates to a quartz glass substrate to be used, for example, as a template substrate for nanoimprint, a photomask substrate for semiconductor lithography such as a Levenson phase shift photomask or a chromeless phase shift photomask, or a MEMS substrate such as a microreactor or a biochip.
2. Discussion of Background
With respect to the configuration of the convexoconcave patterns to be formed on a glass substrate surface, various types such as linear convexoconcaves and circular convexoconcaves are conceivable depending upon the particularly purpose. Typical patterns are shown in FIGS. 1(a), 1(b) and 1(c). As a method for forming such convexoconcave patterns, a method employing a photosensitive organic film and etching i.e. a so-called photolithographic process is common. For example, a method by a photolithographic process employing a positive photosensitive organic film will be briefly described below.
1) A glass substrate having a flat and smooth surface is prepared.
2) A photosensitive organic film (so-called photoresist) is formed on the surface of the glass substrate.
3) The photosensitive organic film is irradiated with high energy rays such as electron rays, X-rays or ultraviolet rays (wavelength: 180 to 400 nm) only at the portions corresponding to the final concave patterns of the substrate to have the desired portions of the organic film exposed. Then, the exposed organic film is removed by means of e.g. an alkaline solution or oxygen gas to form convexoconcave patterns in the organic film on the glass (the glass is exposed at the portions corresponding to the final concave patterns of the substrate, and the portions corresponding to the convex patterns are covered with the organic film).
4) The glass substrate is left in an atmosphere of a reagent (such as hydrofluoric acid, ammonium fluoride or potassium hydroxide) or gas (such as a fluorine compound gas such as F2, SF6, CHF3, CF4 or NF3, or a chlorine type gas such as CHCl3) which is capable of etching the glass substrate, whereby the glass will be eroded (etched) only at the portions where the glass is exposed without being covered with the organic film, and the convexoconcave patterns of the organic film will be transferred to the glass substrate surface.
5) The organic film on the convex portions of the glass substrate surface is removed by means of e.g. an alkaline solution, an ozone gas or a mixed liquid of sulfuric acid with an aqueous hydrogen peroxide solution.
The dimensions of the convexoconcave patterns on the glass substrate formed by such a method are desired to be as close as possible to the expected designed dimensions, and in a case where a plurality of patterns of the same configuration are to be formed on a glass substrate, such pattern dimensions are desired to be uniform as much as possible within the glass substrate. In order to prepare a glass substrate having convexoconcave patterns having such a desired dimensional accuracy, two points are important i.e. to form patterns with high accuracy in the organic film in the above step 3) and to control the etching rate of the glass substrate and its uniformity in the above step 4). Here, the dimensional accuracy of the convexoconcave patterns is required to be considered with respect to three directions. In the case of dry etching of a glass substrate by means of a corrosive gas, for the dimensional accuracy in a horizontal direction (the dimensional accuracy in the X- and Y-directions shown in FIG. 1(a)), the process control of the former (the above step 3)) is particularly important, and for the dimensional accuracy in a vertical direction (the dimensional accuracy in the Z-direction shown in FIG. 1(a)), the process control of the latter (the above step 4)) is particularly important. Further, in the case of wet etching by means of a corrosive reagent, the process control of the above step 4) is influential over the pattern dimensional accuracy in the three directions.
Heretofore, various attempts have been made to make the etching rate uniform. For example, Patent Document 1 discloses that in plasma etching, the non-uniformity in the etching rate caused by the shift of the surface potential of the substrate resulting in the vicinity of the interface between the etching portion and the non-etching portion (the etching mask portion) can be resolved by using a substrate having a thin thickness. Further, Patent Document 2 proposes that an etching mask of the same material as the substrate is used in order to improve the uniformity of the etching rate which is likely to be deteriorated by non-uniformity of the surface potential of the substrate caused by the difference in the material of the surface between the etching portion and the non-etching portion. However, in either method, due to the non-uniformity of the etching rates specific to the materials, it has been difficult to make the etching rate uniform.
Patent Document 1: Japanese Patent No. 03684206
Patent Document 2: Japanese Patent No. 3319568
The etching rate of a quartz glass substrate is not necessarily uniform, and there has been a problem such that due to variation in the etching rate within a substrate or between substrates, the dimensions of convexoconcave patterns formed on the glass substrate vary, and particularly in the case of dry etching which is currently most common as an etching method, the dimensional accuracy in a vertical direction varies. With respect to such dimensional variation in a vertical direction of convexoconcave patterns, for example, in the case of a substrate to be used for e.g. a Levenson phase shift photomask or chromeless phase shift photomask for semiconductor photolithography, the degree of the phase shift influential over the resolution performance in a lithography process, depends on the concave pattern dimension in a vertical direction (the depth of concave).
Accordingly, it is desired to control the dimensions of convexoconcave patterns in a vertical direction to be uniform with accuracy as high as possible and over the entire substrate surface. Thus, variation in the dimensions in a vertical direction of convexoconcave patterns attributable to the non-uniformity of the etching rate of a glass substrate, is a serious problem.